Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter, or “soot”. In addition to particulate matter, internal combustion engines including diesel engines produce a number of combustion products including hydrocarbons (“HC”), carbon monoxide (“CO”), nitrogen oxide (“NOx”), and sulfur oxide (“SOx”). Aftertreatment systems may be utilized to reduce or eliminate emissions of these and other combustion products.
FIG. 1A shows a block diagram providing a brief overview of a vehicle powertrain. The components include an internal combustion engine 20 in flow communication with one or more selected components of an exhaust aftertreatment system 24. The exhaust aftertreatment system 24 optionally includes a catalyst system 96 upstream of a particulate filter 100. In the embodiment shown, the catalyst system 96 is a diesel oxidation catalyst (DOC) 96 coupled in flow communication to receive and treat exhaust from the engine 20. The DOC 96 is preferably a flow-through device that includes either a honeycomb-like or plate-like substrate. The substrate has a surface area that includes (e.g., coated with) a catalyst. The catalyst can be an oxidation catalyst, which can include a precious metal catalyst, such as platinum, for rapid conversion of hydrocarbons, carbon monoxide, and nitric oxides in the engine exhaust gas into carbon dioxide, nitrogen, water, or NO2.
The treated exhaust gases can then proceed to the particulate filter 100, such as a diesel particulate filter (DPF) 100. The DPF 100 is utilized to capture unwanted diesel particulate matter from the flow of exhaust gas exiting engine 20, by flowing exhaust across the walls of DPF channels. The diesel particulate matter includes sub-micron sized solid and liquid particles found in diesel exhaust. The DPF 100 can be manufactured from a variety of materials including but not limited to cordierite, silicon carbide, and/or other high temperature oxide ceramics. The DPF 100 also includes at least one catalyst to catalyze the oxidation of trapped particulate and/or exhaust gas components. For example, the catalyst may include a refractory metal oxide with platinum group metal, although any known oxidation catalyst may be used.
System 24 can include one or more sensors (not illustrated) associated with components of the system 24, such as one or more temperature sensors, NOx sensor, oxygen sensor, mass flow sensor, and a pressure sensor.
The exhaust aftertreatment system 24 can further include an optional Selective Catalytic Reduction (SCR) system 104. The SCR system 104 includes a catalytic surface which interacts with NOx gases to convert the NOx gases into N2 and water. The overall reactions of NOx reductions in an SCR are shown below.4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3→7N2+12H2O  (2)2NH3+NO+NO2→2N2+3H2O  (3)
As shown in Equations (1), (2), and (3), reduction of NOx gases into nitrogen and water requires an ammonia reductant. Thus, a gaseous reductant, such as anhydrous ammonia, aqueous ammonia, or urea, is added (e.g., dosed) to a stream of exhaust gas in a urea decomposition reactor 102 (e.g., a urea decomposition pipe) upstream of an SCR system 104.
FIG. 1B shows an expanded view of urea decomposition reactor 102. As an engine exhaust 204 flows through urea decomposition reactor 102 (as shown, a urea decomposition pipe) towards a mixer 206, a doser 208 injects a reductant 210 in the direction of mixer 206, such that the reductant can be uniformly mixed with the engine exhaust to reduce NOx gases present in the engine exhaust. The dosing frequency and amount of reductant can be adjusted depending on a detected amount of NOx and the engine exhaust temperature.
Urea can be used as a portable and convenient source for ammonia (NH3) reductant in engine aftertreatment systems for decreasing (e.g., eliminating) NOx emission from diesel engines. A two-step thermal process drives the stoichiometric decomposition of urea to produce NH3 in urea decomposition pipe 102: thermolysis of urea into HNCO and NH3, followed by hydrolysis of HNCO (isocyanic acid) into CO2 and NH3, as shown in Scheme 1.

However, both thermodynamic and kinetic limitations can decrease the conversion yield of urea to ammonia. Because the urea conversion requires a relatively long residence time within urea decomposition pipe 102, conversion of urea to ammonia is often incomplete, resulting in overdosing of urea and release of NH3, HNCO, and/or urea into the atmosphere (also known as a NH3, HNCO, and/or urea slip).
Without wishing to be bound by theory, it is believed that the byproduct of incomplete urea decomposition is primarily isocyanic acid (HNCO), a relatively stable gas that rapidly hydrolyzes at the SCR catalyst surface to release more NH3. Thus, isocyanic acid competes with the NOx conversion reaction at the SCR catalyst surface when NOx concentration in the SCR is at or near the peak level. The NH3 that results from undetected urea byproduct decomposition can potentially combine with NH3 from freshly dosed urea to cause an overabundance of NH3 (i.e., overdosing), which results in an apparent failure of the SCR system. Overdosing is rendered even more likely because commercially available NOx sensors can cross-react with NH3 at the tailpipe to report it as NOx, thereby providing a falsely high NOx reading. Thus, two contributing factors can cause apparent SCR failure: (1) unaccounted NH3 from HNCO hydrolysis due to urea byproduct decomposition; and (2) the NOx sensor's inability to distinguish between NH3 and NOx. Each of these two factors, alone or in combination, can lead an engine management system to increase the supply of urea, resulting in overdosing.
In addition to HNCO from incomplete urea decomposition, tar-like compounds are produced from urea byproducts at around 400° C. and above in an engine aftertreatment system (e.g., during a DPF regeneration procedure at up to about 600° C.). These highly undesirable materials can accumulate in the SCR and contribute to premature catalyst aging. Furthermore, during decomposition of pure urea and urea byproducts, very fine particulate matter are released and transported downstream by exhaust flow. The particulate matter can contribute to catalyst fouling/blinding, overdosing of NH3, detection of SCR failure, and/or premature catalyst aging.
Furthermore, one important and persistent consequence of incomplete decomposition of urea is the occurrence of side reactions that form high molecular weight solid deposits, which in turn can have deleterious effects for SCR operation, engine performance, fuel efficiency, and impact system configuration and vehicle design. Deposit formation can vary with engine aftertreatment configuration, and can limit the degrees of freedom available for aftertreatment system and vehicle designers. While urea byproducts can decompose at high temperatures, efforts to decrease solid deposits can fail because at the high temperature of an engine exhaust, conditions that favor decomposition of a specific species in the deposit paradoxically also accelerate the formation of even higher molecular weight compounds within the deposit.
Reductants other than urea can be used to decrease (e.g., eliminate) solid deposit formation. For example, ammonia gas may be used to decrease solid deposit formation. However, compared to urea, ammonia gas is less portable and challenging to provide in a national or international commercial setting. As another example, metal amine complexes can be used to provide NH3. However, metal amine complexes are prohibitively costly for general use and have been limited to niche applications. A variety of portable reactors (e.g., urea hydrolysis devices generate NH3 from urea via electrochemical oxidation) have also been proposed for generating NH3 during vehicle operation, but these reactors are costly, require additional power sources on-board a vehicle, increase overall weight of a vehicle, and/or are bulky.
Accordingly, there is a need for simple and cost-effective methods of generating ammonia from urea with increased yield and efficiency, and with little to no solid deposit formation in engine aftertreatment systems.